DE 41 37 829 C2, which corresponds to U.S. Pat. No. 5,342,241, illustrates a torque limiting coupling with a coupling hub and a coupling sleeve centered around one another on a longitudinal axis. The coupling sleeve includes a circular-cylindrical bearing bore which rotatably supports the coupling hub by its circular-cylindrical outer face. Circumferentially distributed, slot-like first recesses, which are each delimited by two parallel guiding faces, are provided on the outer face of the coupling hub. The slot like recess form a setting axis and start from the outer face. The slot-like recesses each accommodate a first driving member which is adjustable along the setting axis. The driving members include parallel planar contact faces which are guided between the two parallel guiding faces. The driving members can be set to assume a torque transmitting position where the driving members project beyond the outer face of the coupling hub. In a disconnected position, the driving members are retracted into the coupling hub. For the one and only engaged driving direction of rotation, each driving member, towards its head, includes a torque transmitting face which adjoins the first contact face. In the torque transmitting position, each driving member projects beyond the outer face of the coupling hub. Each driving member is in contact with a corresponding supporting face of a groove of the coupling hub. Towards the first contact face, the torque transmitting face assumes an obtuse angle. The supporting face in the groove is inclined in the same way. The side face of the groove, positioned opposite the supporting face, extends in a manner corresponding to that of the second contact face, which is parallel to the first contact face. In the driving direction of rotation, the supporting face is preceded by a chamfer. The above-described design applies to all grooves in the coupling sleeve. The number of grooves corresponds to the number of driving members. A supporting member, loaded by a spring, is arranged in each recess at the driving member end facing away from the supporting face. Each driving member and each supporting member includes two faces which are arranged at different angles. The two faces are arranged in pairs and correspond to one another. The two faces serve to support the driving member in the torque transmitting position. The two faces load the driving member in the disconnected position by a force which is lower in the direction towards the torque transmitting position than in the torque transmitting position.
The torque limiting coupling enables an implement to be driven only at the torque specifically designed for the implement. If an agricultural implement, whose driveline starting from the power take-off shaft of a tractor includes such a torque limiting coupling, is subject to clogging, the driving members, because of the arrangement of the torque transmitting faces and the supporting faces, retract from the torque transmitting position into the disconnected position. The coupling hub can rotate faster in the driving direction of rotation than the coupling sleeve. Only when the rotational speed of the coupling hub relative to the coupling sleeve drops below a predetermined speed is there enough time available to allow the driving members to return into the grooves when the coupling hub rotates relative to the coupling sleeve. The disconnecting function is provided in only one driving direction of rotation.
The torque limiting couplings in DE 32 05 513 C1, which corresponds to U.S. Pat. No. 4,468,206, provide two diametrically arranged driving members that are loaded by a centrally arranged supporting cam. The grooves in the coupling sleeve and the driving member portions entering the grooves are symmetric. The grooves and driving members are both inclined at an angle relative to the adjoining contact face. Thus, a torque limiting function exists in both directions of rotation. However, the disadvantage of this assembly is that such a torque limiting coupling operates like a ratchet. Accordingly, when the coupling hub rotates relative to the coupling sleeve, during overloading, the driving members frequently enter and leave the torque transmitting position. This is particularly disadvantageous in implements operating at high torque values because of the damage that results in premature wear to the edges of the grooves in the coupling sleeve in the region of transition and to the bearing bore. However, a torque limiting function is provided in both possible driving directions of rotation.